Added pillar.auth.cors.allow() decorator

Use this decorator on Flask endpoints that should respond with CORS
headers. These headers are sent in a reply when the browser sends an
`Origin` request header; for more info see [1].

This commit rolls back the previous commit (0ee1d0d3), as this new
approach with a separate decorator is both easier to use and less
error-prone.

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS

pillar/auth/cors.py

@@ -0,0 +1,48 @@
+"""Support for adding CORS headers to responses."""
+
+import functools
+
+import flask
+import werkzeug.wrappers as wz_wrappers
+import werkzeug.exceptions as wz_exceptions
+
+
+def allow(*, allow_credentials=False):
+    """Flask endpoint decorator, adds CORS headers to the response.
+
+    If the request has a non-empty 'Origin' header, the response header
+    'Access-Control-Allow-Origin' is set to the value of that request header,
+    and some other CORS headers are set.
+    """
+    def decorator(wrapped):
+        @functools.wraps(wrapped)
+        def wrapper(*args, **kwargs):
+            request_origin = flask.request.headers.get('Origin')
+            if not request_origin:
+                # No CORS headers requested, so don't bother touching the response.
+                return wrapped(*args, **kwargs)
+
+            try:
+                response = wrapped(*args, **kwargs)
+            except wz_exceptions.HTTPException as ex:
+                response = ex.get_response()
+            else:
+                if isinstance(response, tuple):
+                    response = flask.make_response(*response)
+                elif isinstance(response, str):
+                    response = flask.make_response(response)
+                elif isinstance(response, wz_wrappers.Response):
+                    pass
+                else:
+                    raise TypeError(f'unknown response type {type(response)}')
+
+            assert isinstance(response, wz_wrappers.Response)
+
+            response.headers.set('Access-Control-Allow-Origin', request_origin)
+            response.headers.set('Access-Control-Allow-Headers', 'x-requested-with')
+            if allow_credentials:
+                response.headers.set('Access-Control-Allow-Credentials', 'true')
+
+            return response
+        return wrapper
+    return decorator